Here is the sequence of steps used to perform a partial update on a
document:

The client sends an update request to Node 1.

It forwards the request to Node 3, where the primary shard is allocated.

Node 3 retrieves the document from the primary shard, changes the JSON
in the _source field, and tries to reindex the document on the primary
shard. If the document has already been changed by another process, it
retries step 3 up to retry_on_conflict times, before giving up.

If Node 3 has managed to update the document successfully, it forwards
the new version of the document in parallel to the replica shards on Node 1
and Node 2 to be reindexed. Once all replica shards report success,
Node 3 reports success to the coordinating node, which reports success to
the client.

When a primary shard forwards changes to its replica shards, it doesn’t
forward the update request. Instead it forwards the new version of the full
document. Remember that these changes are forwarded to the replica shards
asynchronously, and there is no guarantee that they will arrive in the same
order that they were sent. If Elasticsearch forwarded just the change, it is
possible that changes would be applied in the wrong order, resulting in a
corrupt document.